1. Field of the Invention
This invention relates to a welt forming device, and more particularly to a sewing machine which is so designed that a welt fabric is sewed to the base fabric.
2. Related Art
Heretofore, a welt for ornamenting, for instance, the edge of the opening of a pocket is formed according to a procedure shown in FIGS. 21(a) to 21(c).
First, a welt fabric W2 is placed on a base fabric W1, and a welt fabric ruler R T-shaped in section is set on the welt fabric W2 from above. Under this condition, both end portions of the welt fabric W2 are folded over along the ruler R thus set as shown in FIG. 21(a), and then the base fabric W1 and the welt fabric W2 are sewed together with sewing needles N. Thereafter, a knife is used to cut the base fabric W1 and the welt fabric W2 at the middle, as viewed in the direction of width thereof, to form an opening (or a slot) as shown in FIG. 21(b). The fabrics W1 and W2 thus sewed are removed from the sewing machine, and both end portions of the welt fabric W2 are passed through the opening 1a and then folded over the rear surface of the base fabric W1. That is, as shown in FIG. 21(c), the welt fabric W2 thus folded form welts which ornament the opening of the pocket.
In each of the welts thus formed, the part Wb of the pocket where the welt fabric 2 and the base fabric W1 are sewed together (hereinafter referred to as "a sewed part Wb", when applicable) is different from the part Wa of the pocket where the welt fabric W2 is exposed (hereinafter referred to as "an exposed part Wa", when applicable) in the number of layers of fabric (four layers of fabric in the sewed part Wb, and two layers of fabric in the exposed part Wb). Those two parts Wa and Wb are shifted from each other as much as T, which impairs the external appearance of the welts. In addition, it is difficult to uniformly press the welts thus formed.
In order to overcome the above-described difficulties, a method of forming welts as shown in FIG. 22(a) has been proposed in the art. In each of the welts, the sewed part WA where the base fabric W1 and the welt fabric W2 are sewed together is equal in the number of layers of fabric to the exposed part WB where the welt fabric W2 is exposed, and the surface of the base fabric W1 is flush with the surface of the welt fabric W2. The welts are formed by using a device as shown in FIG. 23. The conventional welt forming device is disclosed in U.S. Pat. No. 5,333,563 or PCT/JP91/00906.
The device has a pair of fabric receiving stands 1 which are moved back and forth and right and left on the upper surface of the sewing machine table. First, a base fabric W1 is placed flat on those receiving stands 1. Next, a pair of base fabric retainers 2 which are vertically movable to and from the fabric receiving stands 1, are moved downwardly onto the base fabric W1, so that the right and left end portions of the base fabric W1 are held by the right and left base fabric retainers 2 and the right and left fabric receiving stands 1.
Under this condition, the base fabric retainers 2 are slightly (about 0.5 mm) moved towards each other so that the base fabric W1 is slackened as indicated at W1a in FIG. 23. Next, a pair of welt fabric rulers 3 are set between the base fabric retainers 2 from above. Under this condition, two welt fabrics W2 are laid over the base fabric retainers 2 and the welt fabric rulers 3, respectively-one of the welt fabrics W2 is laid over one of the retainers 2 and one of the rulers 3, while the other W2 is laid over the other retainer 2 and the other ruler 3.
Under this condition, a pair of right and left retaining arm members 4, which are vertically movably provided above the base fabric retainers 2, are moved downwardly, so that the welt fabrics W2 are brought into contact with the right and left portions of the base fabric W1 near the needle drop positions P of the needles N as shown in FIG. 23.
Under this condition, a sewing-operation starting instruction is issued. In response to the instruction, the two sewing needles N held by the sewing machine arm start vertical motion while the pair of base fabric retainers 2 are moved forwardly (in a forward direction perpendicular to the surface of the drawing) in synchronization with the vertical motion of those needles N, so that the right and left portion of the base fabric W1 and the welt fabrics W2 are sewed together with seams S1 (first seams) as shown in FIG. 22(b) (only left portion of the base fabric W1 shown in FIG. 22(b)).
Thereafter, the base fabric W1 and the welt fabrics W2 together with the base fabric retainers 2 are returned to their initial positions. Under this condition, the base fabric retainers 2 are moved away from each other to stretch the base fabric W1 flat, removing the slack W1a therefrom.
After the retaining arm members 4 are retracted upwardly, a pair of folding members 5 which are attached to the base fabric retainers 2, respectively, are moved upwardly. Then protruding members 6 which are coupled to the welt fabric rulers 17, respectively, are protruded outwardly, so that first end portions W2a of the welt fabrics W2 are folded along the first seam S1 formed before, respectively, while second end portions W2b are folded over the first end portions W2a, respectively as shown in FIG. 24, and FIGS. 22(c) and 22(d).
Next, the protruding members 6 are retracted, and the retaining arm members 4 are moved downwardly to retain the welt fabrics W2. Under this condition, a second sewing operation is started. In this case, the base fabric W1 is held stretched flat (not being slackened at all). Hence, the second seams S2 formed in this sewing operation are shifted as much as a slight distance H (generally of the order of 0.5 mm) from the first seams S1, respectively as shown FIG. 22(a).
Thereafter, the central portion of the base fabric 1 which is located between the first and second seams is cut with a knife to form an opening (or a slot) therein. Next, the welt fabrics W2 and W2 sewed to the base fabric W1 are turned about 180.degree. about the first seam, respectively. As a result, as shown in FIG. 22(a), the sewed part and the exposed part of the base fabric W1 and the welt fabrics W2 are equal in the number of layers of fabric, and the surface of the base fabric W1 and the surfaces of the welt fabrics W2 and W2 are flush with one another. That is, the welts thus formed are excellent in external appearance, and can be pressed with ease.
As was described above, with the conventional welt forming device, in forming the first seam, the base fabric W1 1 is slackened; and in forming the second seam, the base fabric W1 is stretched, so that the first seam and the second seam are slightly, as much as H, spaced from each other.
Hence, in the case where the base fabric W1 is accurately slackened, the first seam X are formed at the correct positions, and the resultant welts are satisfactory in quality. However, depending on the material or thickness of the base fabric W1, sometimes the latter W1 may not follow the base fabric retainers 2 and the fabric receiving stands 1; that is, the base fabric 1 may not accurately slackened. In this case, it is impossible to form the first seam at the correct positions.
For instance in the case where the base fabric W1 is thick, or stiff, the base fabric W1 cannot follow the movement of the fabric receiving stands 1; that is, it is not sufficiently slackened. As a result, the first seam are shifted from the aimed positions, and therefore the resultant welts are unsatisfactory in quality. In forming the welts, the positions of the first seam are important for determining the width of the welts and the positions of the end portions of the latter. If the positions of the first seam are shifted from the aimed positions, then the opening ornamented with the welt fabrics may be kept open at all times, or the welt fabrics are kept laid over each other on both sides of the opening; that is, the resultant product is poor in external appearance.